I don't know how long I might have done this first without noticing, but here we go.

You can squeeze out an extra casting of spell by a game bug. Let's say I cast Animate Dead, my sorc has 4 castings. So, I start the first casting, I see the guy do the animation, I immediately select the spell again, click on another part of the ground so it "stacks" in a way that once the first is finished, he starts this new. Rince repeat everytime and hey! What's this? I got 5 undead from 4 spells? The last spell if you select it again before it has been cast, gives a double. Talk about gameflaw.

Another (minor) exploit. As we know, if you pick something from the groung while invisible, your invisibility is canceled. This is quite annoying if you just summoned a horde of helpers, then go invisible, let them do the trashing and you just want to loot the ground. Well, I found one way, do not LEFT click the heap of items on ground, RIGHT click it, so your character moves over the pile, then access your inventory and it shows the ground pile in the discarded slots, just pick up what you need, and there you go. Looting while invisible.

Are these old as sky, or did I really stumble into something quite new?

Very old stuff.
That extra spell casting trick is also mentioned in one of the UPPs. probably the JUPP.

:\ Never read them. And I thought I was smart to notice them. Oh well...

I think you were smart to notice them. It would take forever to read all the old posts.